I just bought 3 120mm fan for my Antec 300.
I know 2 fan in front needs to be sucking air in but what about side vent?
Suck air in or out? Btw where can I get some extra filter like 2 in front of case?
You want it taking air in to the GPU area, definitely. If you can put a filter between the fan and the case or even cut a 120mm square of some old nylon stocking it will help stop dust getting in.
In my experience (WARNING: Incoming anecdotal evidence!), the best way to avoid dust is to avoid a vacuum - and the easiest way to avoid a vacuum is by having a greater intake volume by using more intake fans than exhaust. So I'd put the fan on the side as an intake, myself.
TJHarlow, a PC builder did on youtube, made a video in which he tested whether the side fan cools better as an intake or exhaust. He found that it did indeed perform better as an exhaust. Also, on the Antec 300 there is no dust filter, which means that you should definitely use it as an exhaust.